1. Field of the Invention
The present invention relates to an ac power supply device.
2. Prior Art
Power supply devices using the voltage type PWM inverter have been known in the art as e.g. Constant Voltage Constant Frequency (CVCF) type power supply device. With such devices, the inverter effects on/off operation of a dc voltage delivered from a dc power supply, thereby to generate an output voltage which is pulse width-modulated in a sinusoidal form. Since this output voltage includes a large number of higher harmonics, a smoothing reactor and a capactior for absorbing higher harmonics connected to the output of the inverter are used to improve the waveform of the output voltage to deliver an output to a load as a sine wave voltage.
On the other hand, the output voltage is electrically insulated by a transformer and the amplitude of a secondary output of the transformer is detected by a rectifier. The amplitude thus detected as well as a voltage reference is delivered to a voltage controller and the both values are subjected to comparative control. In accordance with an output of the voltage controller, a pattern PWM signal generator generates a PWM signal sinusoidally modulated to carry out on/off control of the inverter.
However, since the above-mentioned conventional system is adapted so that on/off signal is controlled by feedback of a rectified output voltage without controlling instantaneous value of the output voltage, only the amplitude of the output voltage can be controlled, with the result that waveform distortion cannot be controlled. Accordingly, it is difficult to obtain a sine wave output voltage having less waveform distortion. A further drawback with this system is that output waveform varies depending upon the load. This is problem when particularly non-linear load such as rectifier load is connected. The waveform distortion of the output voltage has bad influence on a load such as an electronic computer connected to the output.
For this reason, a filter is used to eliminate waveform distortion. By increasing the number of stages of the filter, waveform distortion can be desirably eliminated accordingly.
However, when a number of filters are inserted, the total time constant of the power supply device becomes large, thus failing to realize high response control.
In view of this, there has been proposed a system to effect control of instantaneous value of the output voltage. According to this system, an output voltage detected is compared with a sine wave reference with the output voltage being in the form of ac to realize instantaneous value control.
Namely, the output voltage of the inverter device is detected by a transformer and the output voltage thus detected as well as a sine wave reference undergoes comparative control in the voltage controller with the output voltage being in the condition of ac signal. The output of the voltage controller becomes an output current reference of the inverter. The output current reference and an output current detected by a current transformer are subjected to comparative control. In accordance with an output of the current controller, the PWM signal generator controls on/off operation of the inverter.
There is proposed another method to effect voltage control at dc level on the basis of rectified signal of the output voltage to obtain a sine wave signal having an amplitude reference as the output thereby to control an output current of the inverter. With this method, the output current becomes sinusoidal, but an output voltage depends upon load. For this reason, particularly in the case of non-linear load such as rectifier load, the output current does not become sinusoidal. Accordingly, it is necessary to effect comparative control of the output voltage using ac signal.
When an instantaneous value of the output ac voltage is thus controlled, the output ac voltage varies by a phase angle of 180 degrees at a frequency more than resonant frequency by reactor and capacitor on the output side. As a result, this causes oscillation, resulting in unstable control. Accordingly, it is unable to improve response of the control system e.g. to cause the gain of the amplifier to be high, giving rise to the problem that the output voltage is distorted with respect to the sine wave reference. An attempt to increase the resonant frequency results in a decrease in the filter constant (L and C), thus making it difficult to suppress higher harmonics.